Designers of integrated circuits such as application specific integrated circuits (ASICs) face growing market pressure to design and deliver their products to customers rapidly. Increasingly, IC manufacturers do not have time to design, evaluate, manufacture, and qualify every sub-circuit of their ICs to customer requirements. Accordingly, they rely on third-party suppliers of sub-circuit designs and layouts. Rights to use such third-party circuitry are typically acquired through licensing of the intellectual property to the sub-circuits from the third-party suppliers.
Third-party IC sub-circuits provide many advantages over in-house custom sub-circuit designs. For example, an input or output (I/O) cell that provides the function of interfacing the IC to an external circuit typically needs to meet certain requirements, such as industry standards (e.g., USB 2.0, IEEE 1394, IEEE 802.3, etc.) or specific performance requirements from a customer. IC manufacturers may not have the expertise or the opportunity to develop such an I/O cell with sufficiently fast time-to-market. Indeed, development of a customized I/O cell requires substantial cost, time, and effort. A third-party I/O cell may have already been developed, tested, and fully qualified to meet the industry standard or to perform according to a range of specifications that includes the customer requirements.
Recognizing that IC manufacturers may have specific performance requirements, third-party suppliers of I/O cells typically provide a limited set of pre-defined programmable or otherwise adjustable performance settings for their I/O cells. For example, a certain I/O cell design includes pre-defined adjustments for operating voltage, drive strength, slew rate, and the like.
IC designers are sometimes confronted with the challenge of having to deliver an IC having I/O cell performance that is outside the standard or adjustable range provided by certain third-party I/O cells. One particular challenge faced by IC manufacturers is providing an I/O cell with increased drive strength, or decreased drive strength, beyond the drive strength available in the I/O cell. Even when the IC manufacturer has rights to an I/O cell with a configurable drive strength setting, the need for configuring the drive strength beyond the available range of drive strength settings may nevertheless exist. Increased drive strength may be needed to drive greater loads or to meet stringent signal timing requirements. Decreased drive strength may be called for when the reactance of the load on the driver circuit is such that rapid signal transitions result in voltage overshoot or undershoot, such as ringing.
Modifying the I/O cell design to meet expanded operating range requirements may not be practicable, or may be prohibited under the applicable intellectual property licensing agreement. Selecting a different third party I/O cell to satisfy needs for an expanded drive strength range may involve having to take on the cost of an additional license for rights to use that I/O cell.
A solution is needed for expanding the range of drive strength settings of certain output circuits without having to undertake the burden of selecting different I/O cells or re-designing the output circuits themselves.